Various types of gyro-stabilized seekers having an imaging optical system arranged on the rotor and imaging a field of view in the plane of a detector are known. The rotor decouples the optical system from the movements of the structure, e.g. of a target seeking missile. As a rule the detector is formed of one single detector element. For increasing the sensitivity this detector is located in a Dewar vessel and is cooled by a Joule-Thomson cooler.
The arrangement of the detector in a Joule-Thomson cooler requires the detector to be structure-fixed, that means, e.g., to be attached to the structure of a missile controlled by the seeker. It is constructively not possible to arrange the Dewar vessel with the Joule-Thomson cooler on the rotor.
The detector supplies target signals which, correspondingly processed as aligning signals, cause the rotor with its axis of rotation to be aligned to a target. With this alignment of the rotor to the target a "squint angle" occurs, that means an angle between the axis of rotation of the rotor and a structure-fixed reference axis, e.g. the longitudinal axis of a missile.
An image processing is necessary for catching and recognizing a target. To this end the observed field of view has to be decomposed into image elements (pixels). With one single detector element this requires an image scanning: The image of the detected field of view has to be movable relative to the stationary detector element. As a rule this is achieved by means of a "rosette-scanning" which is effected by superimposing two gyrating motions. With such a rosette-scanning practically all points of the detected field of view are detected by the detector at least once during each cycle. One of the gyrating motions for generating a rosette-scanning is generally derived from the rotation of the rotor.
The generation of a rosette-shaped scanning path requires an expensive mechanism. Furthermore, the scanning is reatively slow.
Following publications are examples of gyro-stabilized seekers having rosette-scanning: U.S. Pat. No. 4,009,393, U.S. Pat. No. 4,030,807, U.S. Pat. No. 4,039,246, U.S. Pat. No. 4,413,177, U.S. Pat. No. 4,427,878, EP-A-79 684, DE-C-34 38 544 and DE-A-35 19 786.
As a rule, the optical systems are constructed in the manner of a Cassegrain system. They comprise, as primary mirror, an annular concave mirror facing the field of view and, as secondary mirror, a plane mirror facing the detector as well as, in general, additional refracting optical elements. The primary mirror forms the substantial gyrating mass of the rotor.
Linear and two-dimensional arrangement of detector elements are known which are adapted to decompose an image generated thereon into image elements. It is known that such detector arrangements are arranged on chips.
Optical problems arise now when using such not almost punctual detectors in a seeker of the present type. The detector in the Dewar vessel is necessarily structure-fixed. However, the optical system and thus also the image plane in which the image of the field of view and of the target is generated, are pivoted relative to the structure when aligning to the target. This does not matter if the detector comprises a single almost punctual detector element in a central point and the pivoting of the rotor is effected about this central point. However, when the detector is a linear or two-dimensional arrangement of detector elements, then, when a squint angle occurs, the image plane is pivoted relative to the structure-fixed plane of the detector elements. Then a sharp imaging is ensured on a detector element located in the central point. However, the field of view is imaged only obscurely on the other detector elements. These detector elements are not located in the image plane of the optical system.
EP-A-0 100 124 describes an optical seeker having an optical system which is mounted universally pivotable at a structure through a gimbal arrangement and arranged on a gyro rotor and adapted to be aligned to a target.
A detector arrangement is structure-fixedly arranged. A field of view generated in the image plane of the pivotable optical system is transmitted to the structure-fixed detector arrangement through flexible optical fibres.
DE-A-3 435 634 describes a target detecting arrangement for missiles having a detector element rotating about a longitudinal axis of the missile and located behind a lens system, and a zoom lens.